Method for manufacturing prime hot rolled high tensile strength deformed bars

ABSTRACT

The present invention discloses a method for manufacturing high tensile strength deformed bars with an ultimate tensile strength of 500 MPa-700 MPa, comprising the steps of: (a) rolling under heavy reduction rate the common 20MnSi steel billet with an initial rolling temperature of 880˜950° C. through the rough and intermediate rolling mills having an maximum lengthening coefficient μ of 1.449; then passing the rolling workpieces through the pre-water cooling process to decrease the temperature of the workpieces to the range of 850˜950° C.; (b) rolling the workpieces through the finish rolling mill having a main motor power of 1350 KW; then cooling rapidly the surface of the workpieces through the water cooling process having a number of cooling water segments of 1˜5 and a water pressure of 0.8˜1.2 MPa so as to control the surface temperature of the workpieces to the cooling bed within the range of 550˜650° C.; (c) cooling the workpieces through the air cooling bed to cool the core portion of the workpieces; then tempering the workpieces to obtain the finished steel. In accordance with the method of the present invention, the tissue of steel is improved by rolling the common 20MnSi steel billet under heavy reduction rate to manufacture directly the high tensile strength deformed bars with a high tensile strength of 500 MPa-700 MPa so that the manufacturing cost and energy consumption are reduced and the tensile strength of steel is improved, and thereby, the construction cost is decreased.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method for manufacturing steels, and more particularly, to a method for manufacturing prime hot rolled high tensile deformed bars with a tensile strength of 500 MPa-700 MPa.

2. Description of the Related Art

The deformed bar is acted as the framework and the like in the construction of buildings such as industrial factories, reservoirs and dams, road bridges, and so on. Currently, the steel billet of 20MnSi is used by the steel mills to manufacture the HRB335 grade deformed bars. However, the HRB335 deformed bar has a low tensile strength, resulting in the drawbacks of such as large dead weight of the framework structure, high project cost, and long construction period in the construction of especially major projects, and thereby, has already been abandoned for use in construction by developed countries. Nowadays, many alloy elements are added into the steel billet by the steel mills to increase the strength of steel. Namely, alloy elements such as V, Nb, Ti, and so on, are added in the manufacturing of steel billet to realize the purpose of strengthening the tissue of the steel by means of utilizing the solid solution strengthening capability of the alloy elements with other elements in the steel billet. For example, by adding ferrovanadium into the steel, the steel billet of 20MnSiV can be manufactured, and the tensile strength of the steel manufactured accordingly can reach a grade of HRB400. Compared with that of the HRB335 steel, the strength index of the HRB400 steel is increased by 17%. However, the addition of alloy elements complicates the steel manufacture process, and thereby, increases the manufacturing cost.

SUMMARY OF THE INVENTION

In view of the above-described drawbacks, it is an objective of the present invention to provide a method for manufacturing a prime hot rolled high tensile strength deformed bars with an ultimate tensile strength of 500 MPa-700 MPa by utilizing the common 20MnSi steel billet directly so as to realize the purpose of decreasing the manufacturing cost and energy consumption, enhancing the steel strength, saving steel consumption, and decreasing project cost accordingly.

To achieve the above objective, there is provided a method for manufacturing the prime hot rolled high tensile strength deformed bars with a tensile strength of 500 MPa-700 MPa, comprising the following steps:

(a) rolling under heavy reduction rate the common 20MnSi steel billet with an initial rolling temperature of 880˜950° C. through the rough and intermediate rolling mills having an maximum lengthening coefficient μ of 1.449; then passing the rolling workpieces through the pre-water cooling process to decrease the temperature of the workpieces to the range of 850˜950° C.; (b) rolling the workpieces through the finish rolling mill having a main motor power of 1350 KW; then cooling rapidly the surface of the workpieces through the water cooling process having a number of cooling water segments of 1˜5 and a water pressure of 0.8˜1.2 MPa so as to control the surface temperature of the workpieces to the cooling bed within the range of 550˜650° C.; (c) cooling the workpieces through the air cooling bed to cool the core portion of the workpieces; then tempering the workpieces to obtain the finished steel.

In accordance with the method of the present invention, the tissue of steel is improved by rolling the common 20MnSi steel billet under heavy reduction rate to manufacture directly the high tensile deformed bars with a tensile strength of 500 MPa-700 MPa. By applying the rolling process under ultra-low temperature, not only the issue of the excessive cooling intensity in the following cooling processes can be avoided so that the desirable temperature of the steel workpieces to the cooling bed can be achieved, but also the energy consumption and cost can be decreased. The pre-water cooling process is added behind the rough and intermediate rolling processes to decrease the surface temperature of the steel workpieces so as to balance the temperature of the workpieces rose in the process of finish rolling resulting from the high speed of the finish rolling mill. Finally, the steel workpieces are cooled through the water cooling process; the remained heat treatment technology is applied to improve greatly the overall mechanical performance of the steel.

In the rolling processes, the cooling effect to the steel is influenced by the number of segments and the water pressure of the pre-water and water cooling processes, and the rolling speed. In accordance with the method of the present invention, to improve further the steel performance, the number of segments in pre-water cooling process is in the range of 1˜3, the water pressure is normal, the rolling speed of said rolling mills is in the range of 4˜15 m/s.

As a result, the method for manufacturing the high tensile strength deformed bars in accordance with the present invention provides the following advantages:

(1) compared with the HRB335 grade steel in GB1499 standard, with the same 20MnSi material, the strength index of the steel of the present invention is improved by 50% to the HRB500 grade so that the manufacture of steel having a high tensile strength and good elongation is realized. (2) by applying the rolling process under ultra-low temperature, the fuel is saved by about 15%, the manufacture cost is reduced largely, and the enterprise profitability is increased accordingly. (3) in the construction of building, the consumption of steel is saved by 15˜30%, the dead weight of the steel structure is decreased largely, the resource utilization is improved, the manufacture cost is decreased, the period of construction is shortened, the consumption of energy and the pollution to the environment is reduced, so that perfect economic and social results are realized.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a process flow diagram of a method for manufacturing high tensile strength deformed bars in accordance with the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The method for manufacturing the high tensile strength deformed bars in accordance with the present invention will be described further hereinafter referring to the related drawing.

FIG. 1 illustrates a method for manufacturing the high tensile strength deformed bars with a tensile strength of 500 MPa-700 MPa in accordance with the present invention, comprising the steps of:

(a) rolling under heavy reduction rate the common 20MnSi steel billet having an initial rolling temperature within the range of 880˜950° C. through the rough and intermediate rolling mills having an maximum lengthening coefficient μ of 1.449, the composition of common 20MnSi steel billet is shown in Table 1; then passing the rolling workpieces through the pre-water cooling process having a number of water cooling segments of 1˜3 and a normal water pressure to decrease the surface temperature of the workpieces to the range of 850˜950° C.;

TABLE 1 Chemical composition of common 20MnSi steel billet (%) Type C Si Mn P S Ceq 20MnSi ≦0.25 ≦0.80 ≦1.60 ≦0.045 ≦0.045 ≦0.52 (b) rolling the workpieces through the finish rolling mill having a main motor power of 1350 KW; then cooling rapidly the surface of the workpieces through the water cooling process having a number of cooling water segments of 1˜5 and a water pressure of 0.8˜1.2 MPa to control the surface temperature of the workpieces to the cooling bed within the range of 550˜650° C.; (c) cooling the workpieces through the air cooling bed to cool the core portion of the workpieces; then tempering the workpieces to obtain the finished steel.

In accordance with the embodiments of the present invention, the parameters of the rolling process can be controlled to manufacture different products as shown in Table 2.

TABLE 2 Parameters of rolling process of three typical steel bars Temperature to finish rolling Initial Pre-water cooling Water cooling process Rolling rolling Water Water (after pre-water Temperature Spec speed temperature No. of pressure No. of pressure cooling) to cooling (mm) Segmentation (m/s) (° C.) segments (MPa) segments (MPa) (° C.) bed (° C.) Embodiment 1 10 4 9~15 880~950 1~2 Normal 1~2 0.8~1.2 850~950 580~650 pressure Embodiment 1 25 1 9~12 880~940 2~3 Normal 1~3 0.8~1.2 850~950 570~650 pressure Embodiment 1 40 1 4~7  880~930 2~3 Normal 2~5 0.8~1.2 850~950 550~630 pressure

In accordance with the embodiments of the present invention, the performance parameters of the typical and practical steels are shown in Table 3, mainly including the parameters of yield point, tensile strength, and elongation that are generally used to characterize the performance of steel. The tissue of metal is ferrite and pearlite, and the grade of the grain size is ≧9.

TABLE 3 Parameters of mechanical performance of the reinforced steel bars of the present invention Typical specs 10 mm 16 mm 25 mm 32 mm Aging Yield Tensile Yield Tensile Tensile Tensile Yield Tensile No time point strength Elongation point strength Elongation stretch strength Elongation point strength Elongation 1 1 day 543 656 30.0 556 673 27.0 545 690 26.1 575 710 28 2 535 651 28.0 547 660 26.2 545 690 24.2 580 715 26 3 536 652 29.6 548 658 26.8 540 690 24.4 585 715 27 4 541 657 29.6 556 668 28.7 535 685 25.0 585 715 26 5 529 648 28.4 557 670 28.5 545 690 25.3 580 705 28 Average 536.8 652.8 29.1 552.8 665.8 27.4 542.0 689.0 25.0 581.0 712.0 26.7 Stdev 5.50 3.70 0.87 4.87 6.50 1.10 4.47 2.24 0.76 4.18 4.47 0.83 1 30 days 530 655 27.8 552 667 25.6 545 685 24.3 570 695 22 2 525 640 26.8 544 660 25.2 545 685 23.5 580 705 21 3 525 645 28.8 554 666 27.0 550 690 22.6 580 705 21 4 535 655 28.2 542 655 28.2 540 680 24.3 580 705 19 5 540 660 28.8 546 660 26.5 540 685 24.0 585 710 20 Average 531.0 651.0 28.1 547.6 661.6 26.5 544.0 685.0 23.7 579.0 704.0 20.6 Stdev 5.83 7.35 0.74 4.63 4.41 1.06 3.74 3.16 0.64 4.90 4.90 0.88

As shown in Table 3, by applying the rolling process under heavy reduction rate and ultra-low temperature, the high tensile strength deformed bars characterized in the main tissue of ferrite and pearlite (P+F) and the yield strength of exceeding 500 MPa is obtained, and meets the requirements of the strength grade of 500 MPa in the newly revised GB1499 standard. 

1. A method for manufacturing high tensile strength deformed bars with an ultimate tensile strength of at least about 500 MPa-700 MPa, comprising the steps of: (a) rolling under heavy reduction rate the common 20MnSi steel billet with an initial rolling temperature of 880˜950° C. through the rough and intermediate rolling mills having an maximum lengthening coefficient μ of 1.449; then passing the rolling workpieces through the pre-water cooling process to decrease the temperature of the workpieces to the range of 850˜950° C.; (b) rolling the workpieces through the finish rolling mill having a main motor power of 1350 KW; then cooling rapidly the surface of the workpieces through the water cooling process having a number of cooling water segments of 1˜5 and a water pressure of 0.8˜1.2 MPa so as to control the surface temperature of the workpieces to the cooling bed within the range of 550˜650° C.; (c) cooling the workpieces through the air cooling bed to cool the core portion of the workpieces; then tempering the workpieces to obtain the finished steel.
 2. The method of claim 1, wherein the number of pre-water cooling segments of said step (a) is in the range of 1˜3, and the water pressure is normal.
 3. The method of claim 1, wherein the rolling speed of said rolling mills is in the range of 4˜15 m/s. 